1. Field of the Invention
The present invention is directed to a process for the production of 4,4'-diamino-dicyclohexylmethane with a low trans-trans isomer content by the catalytic hydrogenation of 4,4'-diamino-diphenylmethane.
2. Description of the Prior Art
In order to obtain a diisocyanate, which is liquid at room temperature, through the phosgenation of 4,4'-diamino-dicyclohexylmethane (4,4'-HMDA) or its mixtures with the 2,4'-and/or 2,2'-HMDA isomers, the content of the trans-trans isomer of 4,4'-HMDA must lie within a certain range. Besides the other geometric isomers which are obtained by the hydrogenation of 4,4'-diamino-diphenylmethane (4,4'-MDA), namely the cis-cis and the cis-trans isomers, the content of the trans-trans isomer of 4,4'-HMDA in the hydrogenated product must be about 15 to 40%, preferably about 15 to 30% and most preferably about 18.5 to 23% by weight, based on the diamine content of the hydrogenated product, in order to subsequently obtain a diisocyanate which remains liquid.
There are a series of known methods by which MDA is hydrogenated with different catalysts without considering the amount of trans-trans isomer which is obtained. As an example, U.S. Pat. Nos. 3,155,724, 3,644,522, and 3,766,272 should be mentioned. In these hydrogenation processes the trans-trans content can reach the thermodynamically theoretical value of 50 to 55% by weight. Thus, the trans-trans content of 4,4'-HMDA in the hydrogenated product has to be reduced from an average of 50 to 55% by weight down to the above-mentioned ranges through an additional separation process. Further, if there are no uses for the trans-trans isomer enriched mixture, the process suffers from low yields and high costs and is therefore an uneconomical way to produce 4,4'-HMDA with a low content of the trans-trans isomer.
Among the different catalysts, e.g., Co or Mn catalysts on inert supports (e.g. U.S. Pat. No. 3,743,677), rhodium/aluminum oxide catalysts on inert supports (e.g. German Patent 2,423,639), iridium/aluminum oxide catalysts on inert supports (e.g. U.S. Pat. No. 3,914,307), ruthenium catalysts on inert supports (e.g. U.S. Pat. Nos. 2,606,925, 2,606,928, 3,347,917, 3,636,108, 3,676,495 and European Patents 66,210, 111,238) or ruthenium dioxide (e.g. U.S. Pat. Nos. 2,494,563, 2,606,924, 3,742,049 and European Patent 66,211), it is known that rhodium and ruthenium catalysts are specifically suited for the hydrogenation of MDA, especially if a high selectivity for 4,4'-HMDA with a simultaneously low content of the trans-trans isomer is desired.
The previous processes which used ruthenium catalysts (see e.g. U.S. Pat. Nos. 2,494,563, 2,606,924, 2,606,925, 2,606,928, 3,347,917, 3,676,495, 3,959,374, 3,743,677, 3,914,917, 3,825,586, 3,636,108, and 4,161,492) resulted in amine mixtures with sufficiently low trans-trans isomer contents. However, the rates of reaction are too slow to yield technically interesting yields of 4,4'-HMDA. More recent processes using ruthenium catalysts (e.g., European Patents 66,210 and 66,211) result in low trans-trans isomer contents with technically interesting rates of reaction, but they have the disadvantage that because of by-product formation the yields of 4,4'-HMDA are low (less than 95%) and also the catalysts have a short lifetime. The same is true for the known processes using rhodium catalysts (e.g., European Patent 66,212); a fact which makes such processes technically uninteresting, notwithstanding the comparatively higher price of rhodium.
In European Patent 111,238 a catalyst containing 5% ruthenium on an inert support which is treated with a nitrate or sulfate of an alkali or alkaline earth metal is described. The use of this catalyst in a slurry process provides a 93% yield of 4,4'-HMDA having a trans/trans content of 23% as reported in Example 1 of that patent. In U.S. Pat. No. 3,636,108 and 3,697,449, other alkali and alkaline earth metals are used with ruthenium supported powdered catalysts to produce catalyst systems which can be used in slurry processes to give high yields of 4,4'-HMDA. Without the use of such promoters, the powdered ruthenium caralysts produce significant amounts of polymeric material. (See the comparison examples in this application and Example 13 in EP 111,238). However, it has been discovered that the fixed bed process described herein produces insignificant amounts of polymerics without the use of promotors using process conditions which are similar to those of the above-cited patent examples. (Also, see Table 1 of this application). Another disadvantage of the slurry process is that the catalyst must be separated from the product and recycled.
It is an object of the present invention to provide a process which does not have the above disadvantages and which is capable of hydrogenating MDA with high selectivity and high catalyst activity, i.e. a process which results in high throughputs of MDA to HMDA containing about 15 to 40 weight percent, preferably about 15 to 30 weight percent, more preferably 18.5 to 23.5 weight percent and most preferably about 23 weight percent of the trans-trans isomer of 4,4'-HMDA, based on the diamine content of the hydrogenated product. It is a further object of the present invention to provide an economical process which is capable of continuous operation, preferably without solvents, and results in a high, if not complete, conversion of MDA. Also, the catalyst should have a long lifetime. These objectives can be achieved according to the present invention as set forth hereinafter.